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Hi all! I’ve been working through some basic lessons in electronics. I’m working on the capacitor section that deals with using capacitors and resistors as a filter. So I set a simple test circuit. I’ve setup my computer to create a sine wave via the sounds card. I then connect a wire from the sound card to my test board. On the test board I have a .22uf capacitor and a 1.5 ohm resistor (+/- 20%). The configuration is attached. Then I connect to my scope. I have tried to setup the capacitor and resistor as both a high and low pass filter. However, what I’m seeing is that in low pass, I’m blocking all (at least all that my 20Mhz scope can see) AC, and in High Pass I’m passing all AC.

My understanding is that with a 1.5Ohm resistor and a .22uf cap I should be dealing with ~3khz. Meaning the low pass should pass anything below that, and the high pass should pass anything above that. However, that isn’t what I see… Is my math wrong? Where am I going wrong?


Below is a basic drawing of my low pass configuration. Thanks for any tips and help!

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High pass and low pass filters both have a critical frequency associated with them.  The critical frequency is when the output voltage drops by 3db, often referred to as the half power point.  The formula to calculate the critical frequency is shown below along with the values from your circuit.

So, however you calculated 3KHz does not seem to be correct.  The frequencies that your low pass will pass go all the way up to 482KHz, maybe you didn’t check frequencies that high since you were looking for something in the 3KHz range.  And then of course your high pass would block everything up to 482KHz and then pass frequencies above that.

It may be possible that you really meant that the resistor is a 1.5K ohm resistor.  If that’s the case then the calculation for your critical frequency would be:

So your low pass filter would pass everything up to 482.29Hz and your high pass filter would block everything up to 482.29Hz and pass everything above that.

I hope this helps you with the understanding of your circuit.  Please reply here if you have further questions.

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